The present invention relates to display systems for use in cooperation with a computer or similar control system. The invention more particularly relates to circuitry for adding a light pen capability to plasma and similar display systems.
Plasma display panels in which light is emitted from an array of individual gas discharge cells are well known in the art. For example, Bitzer et al. U.S. Pat. No. 3,559,190 issued Jan. 26, 1971 describes an early development in the field. Plasma display panels are in some respects similar to well-known cathode ray tube (CRT) arrangements such as those described in W. H. Ninke U.S. Pat. No. 3,653,001 issued Mar. 28, 1972 and R. A. Koster U.S. Pat. No. 3,389,404 issued June 18, 1968. An important difference, however, is that plasma display panels have inherent memory. That is, they need not be constantly refreshed by an information-bearing signal corresponding to the desired visual image.
More particularly, each cell of a plasma display panel includes a volume of ionizable gas. A selected cell is established in the ON (energized, light-emitting) state via a write pulse, the magnitude of which exceeds the breakdown voltage of the gas. A glow discharge is thereby created at the selected cell and a single, short, e.g., one microsecond, light pulse is emitted. The write pulse also stores a "wall" voltage across the gas of the selected cell. Alternating polarity sustain signals, which are constantly applied to all cells of the panel, thereafter combine with the wall voltage to effect successive discharges and to maintain wall voltage storage. The sustain signal frequency may be on the order of 50 kHz so that the individual light pulses emitted by the cell are fused by the eye of the viewer and the cell appears to be continuously light-emitting. The magnitude of the sustain signals is less than the gas breakdown voltage so that cells which have not received a write pulse, and thus at which no wall voltage has been stored, remain OFF (de-energized, non-light-emitting).
A cell in the ON state is returned to the OFF state by applying an erase pulse thereto. The magnitude of the erase pulse is sufficient in combination with the wall voltage stored at an ON cell to cause a breakdown. However, the magnitude and duration of the erase pulse are such that there is no further wall voltage storage. Hence, no further discharges occur.
A useful adjunct to any computer-based display system is a so-called light pen for communicating to the computer or other control mechanism a location on the display surface. In typical CRT display systems, such as those described in the above-mentioned Ninke and Koster patents, a light pen placed on the CRT surface signals the computer or other control device the instant that refresh information applied to the CRT causes the phosphor adjacent to the pen tip to emit light. Data identifying the point on the CRT surface being refreshed at any given time is correlated with the light pen signal to identify the location of the pen on the CRT surface.
Plasma display panels are not usually operated in a sequential scan refresh mode. Rather, all ON cells emit a light pulse, or "flash," at the same time, and in response to the sustain signals applied to all cells. Thus, light pen circuitry for plasma display systems operates somewhat differently. For example, P. D. Ngo U.S. Pat. Nos. 3,851,327 issued Nov. 26, 1974 and 3,967,267 issued June 29, 1976 disclose specially adapted "scan write" and "scan erase" pulses which are applied to the cells of a plasma display panel in pairs at a time when the ON cells of the panel are between sustain-initiated flashes. The scan write and scan erase pulses flash OFF and ON cells, respectively, a single time without altering the cell state. The light pen output is ignored at standard ON cell flash times. However, once the pen signals at a nonstandard time that the cell to which it is adjacent has flashed, the position of the pen on the panel surface is known, since the location of the cell which was most recently flashed is known.
An important factor in designing light pen detection circuitry for plasma display panels is consideration of the typically narrow operating margins for scan signals. This is caused, at least in part, by the fact that the breakdown voltages associated with the cells of a plasma panel vary over a range of values in more or less random fashion. Accordingly, some care must be exercised in the choice of scan pulse amplitude, duration and shape to ensure that spurious writing and erasing of cells does not occur, on the one hand, while ensuring reliable cell flashing, on the other hand.
A light pen detection system with wide operating margins is described, for example, in the above-mentioned '267 patent. In particular, the scan write pulse of that patent is characterized by a magnitude sufficient to create a glow discharge and light flash at an OFF cell. The cell is prevented from switching to the ON state by utilizing a short pulse duration, for example. This minimizes the amount of wall voltage stored in response to the scan write pulse, ensuring that the magnitude of whatever wall voltage is stored, is less than the difference between the breakdown and sustain signal voltages. Hence, the cell is not switched to the ON state by the scan write pulse. The '267 patent also discloses that the storage of wall voltage in response to the scan write pulse can be further minimized by providing the pulse with a tapered, rather than an abrupt, or sharp, trailing edge. This advantageously allows the scan write pulse magnitude and duration to be made sufficiently large to ensure that OFF cells with relatively high breakdown voltages will, in fact, flash while minimizing the danger that an inordinately high wall voltage might be stored at OFF cells having relatively low breakdown voltages, thereby undesirably switching such cells to the ON state.